All functions in the multi interface are prefixed with curl_multi.
1. Enable a "pull" interface. The application that uses libcurl decides where and when to ask libcurl to get/send data.
2. Enable multiple simultaneous transfers in the same thread without making it complicated for the application.
3. Enable the application to wait for action on its own file descriptors and curl's file descriptors simultaneous easily.
Each single transfer is built up with an easy handle. You must create them, and setup the appropriate options for each easy handle, as outlined in the libcurl(3) man page, using curl_easy_setopt(3).
When the easy handle is setup for a transfer, then instead of using curl_easy_perform(3) (as when using the easy interface for transfers), you should instead add the easy handle to the multi handle using curl_multi_add_handle(3). The multi handle is sometimes referred to as a 'multi stack' because of the fact that it may hold a large amount of easy handles.
Should you change your mind, the easy handle is again removed from the multi stack using curl_multi_remove_handle(3). Once removed from the multi handle, you can again use other easy interface functions like curl_easy_perform(3) on the handle or whatever you think is necessary.
Adding the easy handle to the multi handle does not start the transfer. Remember that one of the main ideas with this interface is to let your application drive. You drive the transfers by invoking curl_multi_perform(3). libcurl will then transfer data if there is anything available to transfer. It'll use the callbacks and everything else you have setup in the individual easy handles. It'll transfer data on all current transfers in the multi stack that are ready to transfer anything. It may be all, it may be none.
Your application can acquire knowledge from libcurl when it would like to get invoked to transfer data, so that you don't have to busy-loop and call that curl_multi_perform(3) like crazy. curl_multi_fdset(3) offers an interface using which you can extract fd_sets from libcurl to use in select() or poll() calls in order to get to know when the transfers in the multi stack might need attention. This also makes it very easy for your program to wait for input on your own private file descriptors at the same time or perhaps timeout every now and then, should you want that.
A little note here about the return codes from the multi functions, and especially the curl_multi_perform(3): if you receive CURLM_CALL_MULTI_PERFORM, this basically means that you should call curl_multi_perform(3) again, before you select() on more actions. You don't have to do it immediately, but the return code means that libcurl may have more data available to return or that there may be more data to send off before it is "satisfied".
curl_multi_perform(3) stores the number of still running transfers in one of its input arguments, and by reading that you can figure out when all the transfers in the multi handles are done. 'done' does not mean successful. One or more of the transfers may have failed. Tracking when this number changes, you know when one or more transfers are done.
To get information about completed transfers, to figure out success or not and similar, curl_multi_info_read(3) should be called. It can return a message about a current or previous transfer. Repeated invokes of the function get more messages until the message queue is empty. The information you receive there includes an easy handle pointer which you may use to identify which easy handle the information regards.
When all transfers in the multi stack are done, cleanup the multi handle with curl_multi_cleanup(3). Be careful and please note that you MUST invoke separate curl_easy_cleanup(3) calls on every single easy handle to clean them up properly.
If you want to re-use an easy handle that was added to the multi handle for transfer, you must first remove it from the multi stack and then re-add it again (possibly after having altered some options at your own choice).